i) Field of the Invention
The present invention relates to a soil water detector and more particularly to a device for monitoring water potential within a soil. The present invention also relates to a water potential detector for an irrigation system comprising a probe that allows the monitoring of water content within the soil.
ii) Description of Prior Art
Saving irrigation water is an increasing concern among growers. Actually, reliable soil water content or suction measurements are the basics of a proper irrigation management for saving water and fertilisers. In nursery and greenhouse production, the water available to plants is stored within the potted substrate. As the amount of water remains limited by the volume of the container, additional water therefore needs to be added to the crop through different irrigation systems. Irrigation water is also needed for vegetable and orchard crops to improve yield, since such productions are often grown on coarse and sandy soils to provide a good drainage. However, as a result of this fast drainage, soil water retention is limited. In addition to the fact that the risk of drought increases with global warming, the proper tools used to initiate irrigation are limited.
Usually, irrigation is scheduled based on meteorological data, visual inspection (colour of soil or substrate), appearance of the crop or use of timers. Soil moisture sensors represent an alternative to the more conventional techniques for scheduling irrigation. Sensors are designed to measure the suction forces retaining water within the soil matrix, which is commonly referred as soil water potential. Water potential is defined as the potential energy of water per unit mass of water in the system. The total water potential of a soil is the sum of four component potentials: gravitational, matrix, osmotic, and pressure. Gravitational potential depends on the position of water in a gravitational field. Matric potential depends on the adsorptive forces binding water to a matrix. Osmotic potential depends on the concentration of dissolved substances in water. Pressure potential depends on the hydrostatic or pneumatic pressure on water. Matric potential is a force strong enough to prevent water from being absorbed by the plant and in non saline soil, matric potential is the dominant driving force limiting water availability to plant. Therefore, the measurement of matric potential is very advisable for water and irrigation management.
Different soil water potential sensors have been described in the art. However, the existing devices suffer from limitations. For managing irrigation in organic growing media, such as nurseries or greenhouse crops, time or frequency domain reflectometry systems constitute alternatives. These technologies measure soil water content, a parameter indirectly linked to soil water potential. These technologies however face important reliability problems since the length of the electrodes needed with such technology in small size pot is not sufficient and their price is very high. In addition, it was demonstrated that the signal is sensitive to soil solution having high electrical conductivity. Moreover, these systems require calibration to infer water potential and therefore lack of accuracy, especially in organic growing media. Other types of sensors which directly measure soil water potential are accurate but are very expensive and tricky to use. Indeed, the output signal of these sensors often need a human interpretation and most of these devices have to be routinely calibrated. Finally, most of the proposed sampling devices are not designed to properly start the irrigation and without signal interpretation neglect the adequate interruption of irrigation.
Recently, a simple electro-optical device that may operate in growing media was introduced. This device is based on the use of an infrared light emitting diode and photodiode. The LED diode emits a light signal, which is transmitted in higher amount as the water content increases in a porous translucent body made from nylon. Although this technology appeared promising, nylon filter coupled with LED gave unreliable results. Indeed, the proper operation of a nylon filter in growing media requires pores that are quite large i.e. pores having at least a 30 μm diameter. Moreover, it requires a good contact area, a proper response time and a structure sufficiently strong to allow the insertion of the device into the growing media without loosing contact with the soil. Unfortunately, these important characteristic cannot be reached with the nylon filter. Moreover, it is known in the art that nylon filters provide an output signal that needs interpretation, a feature convenient for researchers but inappropriate for growers.
For example, U.S. Pat. No. 4,899,047 Cry et al. describes a device to detect the presence of oil in water by placing a translucent, porous body of hydrophobic material in contact with the oil and water and detecting the amount by which light incident on the body is attenuated on propagation through the body. This action is carried out using a translucent probe including a porous treated polyethylene material capable of absorbing liquid.
Considering the state of the art described above, it would be highly desirable to be provided with a soil water potential measurement device that can be used to control the initiation and the termination of an irrigation, which is efficient, affordable and easy-to-use.